Apparatus of the general class to which the invention relates is disclosed in U.S. Pat. No. 4,257,766 to Ritzmann et al. In this case the downwardly-inclined part of the tertiary air pipe is constructed as a simple cylindrical pipe.
In order for the fine material from the second lowest stage of the cyclone preheater which is introduced into the tertiary air pipe to be satisfactorily aerated on the short available path (between the opening of the material pipe and the combustion zone) and to be reliably introduced into the combustion zone, it is known from U.S. Pat. No. 4,579,526 to Kreft et al for the downwardly-inclined part of the tertiary air pipe to be narrowed so as to increase the flow speed of the air at the opening of the material pipe and for the air pipe between the narrowed section and the calciner which forms the combustion zone to be constructed as a diffuser.
In the practical operation of such apparatus it has been found that in certain cases (particularly in the event of slowness of reaction of the fuel and/or the raw material) the combustion of the fuel and the deacidification of the raw material in the calciner are achieved to a limited extent only.
The object of the invention therefore is to construct apparatus of the type referred to in such a way that, even in the event of a slow reaction of the fuel and/or raw material, good combustion of the fuel and a high degree of deacidification in the calciner can be achieved.
This object is achieved according to the invention by narrowing the tertiary air pipe at the point where it joins the calciner.